Mobile communications networks are in the process of offering increasingly sophisticated capabilities associated with locating the position of a wireless device. New software applications, such as, for example, those related to personal productivity, collaborative communications, social networking, and data acquisition, may utilize geo-location information to provide new features to consumers. Also, some regulatory requirements of a jurisdiction may require a network operator to report the location of a mobile terminal when the mobile terminal places a call to an emergency service, such as a 911 call in the United States.
In a Code Division Multiple Access (CDMA) digital cellular network, the position location capability can be provided by Advanced Forward Link Trilateration (AFLT), a technique that computes the location of the wireless device from the wireless device's measured time of arrival of radio signals from the base stations. A more advanced technique is hybrid position location, where the mobile station may employ a Satellite Positioning System (SPS) receiver where the position is computed based on both AFLT and SPS measurements.
SPS receivers are being incorporated into wireless devices in order to increase the accuracy of wireless device location determination. The SPS receivers can be autonomous and perform all SPS acquisition functions and position calculations (also known as standalone), or they can be non-autonomous (also known as wireless assisted) and rely on other wireless network technologies for providing SPS acquisition data and possibly performing the position calculations.
In wireless devices having both a wireless communications system and an SPS, it may become cost effective to share system components which can be used by both systems. For example, such systems may share a reference oscillator, such as tunable reference oscillator and/or a free-running reference oscillator for providing reference frequencies. Sharing such components can reduce cost, complexity, size, weight, and power consumption.
However, sharing components may involve some compromises in performance and/or functionality to reduce operation complexity. For example, current wireless devices may allow shared frequency management based only upon the communications system's performance, but not specifically for the SPS's performance.
Position determination accuracy of a wireless communication device may be negatively affected by frequency biases, which in turn may affect SPS Doppler estimations and SPS Doppler measurements conducted by the wireless device. Large unaccounted frequency biases can prevent the wireless device from acquiring satellites. Large unaccounted frequency biases may also result in poor quality SPS Doppler measurements which can also adversely affects position accuracy, because of the adverse effect on the SPS code phase measurement determination.
A major contributor to the complexity associated with searching and acquiring the satellite signal is the frequency error attributable to the receiver Local Oscillator (LO). The LO is used in the receiver to down convert the received signal to a baseband signal. The baseband signal is then processed. In the case of a signal received from a SPS satellite, the baseband signal is correlated to all possible pseudo random codes to determine which satellite transmitted the signal, and to determine the time of arrival of the signal. The search and acquisition process is greatly complicated by the LO frequency error. Any frequency error contributed by the LO creates additional search space that typically should be covered. Furthermore, the LO frequency error presents a separate dimension over which time of arrival may be searched. Thus, the search space is increased in proportion to the frequency error, since the time of arrival search may be conducted over all possible frequency errors. Many parameters contribute to real or perceived LO frequency error. The circuit operating temperature as well as the temperature gradient across the circuit board affects the LO frequency. Additionally, the frequency stability of the reference oscillator used to generate an LO contributes directly to the LO frequency stability.
Accordingly, it would be beneficial to employ a unified frequency management approach for reducing the LO frequency error to reduce the search space covered in baseband signal processing. Reduction in the search space allows for lower search complexity, which in turn allows for greater receiver sensitivity and decreased search and acquisition times.